The blanks are heated by subjecting their bodies to an infrared electromagnetic radiation of appropriate wavelength emitted by infrared incandescent lamps, typically in the general shape of a tube. In practice, the blanks are moved by a conveyor such that their bodies follow a determined path, along which heating lamps are arranged in numbers and positions adapted to the thermal effect to be generated within the thermoplastic material.
It is known to monitor the condition, operational or no operational, of the lamps by using various electrical solutions, the most usual of which consisting of monitoring the intensity of the current supplying them, either individually, or per group. These solutions make it possible to detect when a filament breaks.
However, the lamps used in the installations previously mentioned are halogen atmosphere lamps, which during operation must be kept at a closely defined temperature to ensure that the tungsten particles originating from the filament can be eliminated efficiently by the halogen. If the operating temperature is too high (in the case of insufficient ventilation) or too low (in the case of excessive ventilation), elimination of the tungsten particles by the halogen does not take place, or takes place insufficiently, and tungsten is deposited on the envelope, particularly made of glass or quartz: the envelope then finally disintegrates and breaks.
It has however been noted that breaking of the envelope is not necessarily accompanied by the concomitant breaking of the lamp filament, and there can be a consequent time-lapse (for example several seconds, even several tens of seconds) before the filament breaks (breaking caused by the filament coming into contact with the oxygen in the surrounding air, and its rapid oxidation).
For that reason, the usual means employed to detect breaking of the filament are ineffective for detecting the breaking of the envelope which may precede the breaking of the filament.
Now, breaking the envelope of a lamp is a far from harmless event, and can result in significant damage: the fragments of the envelope, particularly glass or quartz, can destroy adjacent lamps; they can damage the blanks moving past and render them unsuitable for commercial use; they can even lodge in one of these and subsequently remain there, even in the final container; they can damage mechanical parts of the installation and cause jamming in the installation operating at considerable speeds. In all cases, an emergency shutdown of the installation is required, resulting in a loss of production. Finally, the debris of the envelope can cause injury to the staff responsible for repairing the installation.
The current means, the implementation of which is essentially based on the physical condition of the lamp filament, do not make it possible to detect the breaking of the envelope alone, while the filament is still intact.